Composite structures are used for a variety of applications on most aircraft such as wing planks, vertical and horizontal stabilizer skins, fuselage panels, and various other components. The composite structures are typically formed on a mold which is mounted on a base plate. Layers of composite material are impregnated with uncured resin and then laid up over the mold. A vacuum bag is then placed over the uncured composite material and sealed to the plate about the periphery. To cure the part, a vacuum is drawn from between the vacuum bag and the base plate. Subsequently, the assembly is placed in an autoclave where the temperature and pressure are raised to cure the resin. After curing, the assembly is returned to room temperature. The vacuum bag is then removed so that the cured part may be separated from the mold. U.S. Pat. No. 4,284,679 entitled "Filled Resin Coated Tape" by L. Blad describes another process by which composite structures may be formed. Here, strips of resin and syntactic resin are laid into grooves on the surface of a rubber coated mandrel. A set of female cylindrical tools is assembled around the mandrel. A vacuum bag enclosing the female tools is sealed to the rubber surface of the mandrel and a vacuum is drawn. The mandrel is then removed and the sealed structure is placed in an autoclave at 250.degree. F., 100 psi for one hour to cause the resin to cure.
The construction of the vacuum bags used in the above-described processes is time consuming, expensive, and is unreliable unless skilled personnel are assigned to the task. The task is further complicated when the critical point of establishing the airtight seal is also the point of fastening. The requirement of the airtight seal limits both the types of fastening and vacuum bags that may be used. The most common bag material is nylon which is easily damaged so that it usually must be replaced after each cure cycle. Such a design is disclosed in U.S. Pat. No. 4,016,022, entitled "Low Flow, Vacuum Bag Curable Prepreg Material for High Performance Composite Systems". Here, the nylon vacuum bag is sealed to the base plate using a commercial sealant. Another vacuum bag design is disclosed in U.S. Pat. No. 3,553,054, entitled "Laminated Structural Members:" by Maus. A vacuum bag, of polymeric composition, is sealed to the base plate using a zinc-chromate tape adhesive. This type of seal is not reliable.
An alternative material used is silicone rubber. Silicone rubber has an elongation of 650% which makes it possible to stretch it over molds of various sizes and shapes; it can be exposed to a maximum temperature of 500.degree. F. which allows greater curing cycle flexibility; and it is easily repairable which extends the service life. Unfortunately, great difficulty has been encountered in sealing the silicone rubber bag to the base plate using the above-described conventional techniques.
Another vacuum bag design is discussed in U.S. Pat. No. 4,287,015, entitled "Vacuum Bags Used in Making Laminated Products" by H. J. Danner. In this patent a vacuum bag is disclosed, which comprises a closed-loop ribbed structure bonded to a base plate about a mold. A stretchable bag is placed over the entire structure and fastened to the base plate about the periphery of the ribbed structure. The bag has a zipper or velcro type fastener located around the periphery of the vacuum bag and the ribbed structure. Thus, when the bag is "unzipped" the structure to be formed can easily be laid on the mold. Prior to pulling a vacuum, the bag is zipped-up so that a seal is formed between the bag and ribbed structure. This method could be used with silicone rubber but it is an expensive system.
Other sealing techniques include the use of a peripheral groove on a plate into which a vacuum bag is forced. The vacuum bag is held in place by a tubular shaped, resilient material wedged into the groove. This type of seal, however, tends to leak. In addition, there is the expense of machining the groove. Another technique is to place a "picture frame" over the vacuum bag which may then be clamped to the base plate. This technique is subject to leakage, due to warpage of the picture frame, and adds the expense of construction of the picture frame. A final technique is to completely enclose the mold, composite, and base plate between upper and lower sheets of bag material and to seal the two sheets together with conventional bag sealant. This has the obvious disadvantage of doubling the amount of bag material required. Furthermore, if the part to be cured is large, the installation of the lower portion of the bag will be unwieldy.
Therefore, it is a primary object of the subject invention to provide a sealing system utilizing a reusable vacuum bag in the fabrication of composite structures.
It is another primary object of the subject invention to provide a sealing system so that conventional bag sealing methods can be used with silicone rubber bags.
It is a further object of the subject invention to provide a vacuum bag which is both easy to install and easy to remove after the part has been cured.
It is a still further object of the subject invention to provide an inexpensive vacuum bag sealing system.